Method and apparatus for lapping or polishing materials

ABSTRACT

Wafers to be processed are mounted to a lapping plate of a lapping machine using a photosensitive thermoplastic material, such as a photoresist. In a preferred embodiment, the wafers are laminated to a dry film photopolymer disposed on a carrier sheet, after which the sheet is secured to the lapping plate using a pressed-fit hoop that stretches the carrier sheet across the surface of the lapping plate and holds the sheet secure about the perimeter of the plate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to polishing and lapping methods andapparatus and, more particularly, to the mounting of material wafers tothe lapping plate of a lapping machine.

2. Description of the Prior Art

THe polishing and lapping steps utilized in the processing of materialssuch as glass and semiconductor wafers are both time-consuming andcostly operations and when used in semiconductor processing the stepsresult in a high percentage of rejects. During all but the finalprocessing steps for a semiconductor wafer, the wafer is maintained at asubstantial thickness to reduce the likelihood of accidental breakageduring processing and particularly during probe testing of the wafer.Prior to the scribing and breaking steps for dividing the wafer intoindividual semiconductor die, it is essential that the wafer thicknessbe reduced to make scribing and breaking easier. This process iscommonly known as back-lapping wherein a substantial portion of thewafer is removed, by a grinding or lapping operation, from the back-sideof the wafer.

Heretofore, the wafers were mounted face down on a flat lapping platethat usually is circular with a diameter of approximately twelve inches.The wafers were held in place on the lapping plate by a standard waxmounting process wherein the lapping plate was heated to the wax meltingtemperature and a layer of wax was formed on the flat surface, thewafers were then placed on the heated wax and clamped in position whilethe lapping plate cooled and the wax hardened. The excess wax then hadto be removed so that no wax extended above or on the exposed back-sideof the wafers. The backside of the wafers was then ground off using alapping slurry containing an abrasive such as Al₂ O₃ or diamond pastefor removing the semiconductor material.

After completion of the lapping process, the lapping plate and waferswere washed to remove excess lapping slurry and the plate was againheated to melt the wax, and the wafers were then removed from thelapping plate. Due to surface tension, the wafer usually had to be priedor slid off of the plate resulting in substantial damage to the activesurface of the semiconductor wafers. Any sliding of the wafer on thelapping plate tended to scratch the front surface since much of theabrasive material remains embedded in the wax. Since the semiconductordevices are already formed on the front surface of the wafer, any amountof scratching will damage the devices and it is not uncommon toexperience 30 to 50% rejects as a result of the back-lap process. Afterthe wafer is removed from the waxed plate, the wafers must be cleanedwith a solvent solution.

Aside from the substantial number of rejects experienced during thelapping process, the process is also extremely expensive andtime-consuming. The lapping plate must first be heated to melt the wax,then fully cooled to provide good adhesion of the wafers to the plateand then again reheated for removal of the wafers. This procedure is, ofcourse, quite time-consuming. The cleaning operation is also becomingincreasingly expensive as the cost of solvent continues to rise.

Another difficulty that was encountered in the use of wax mounting wasthickness control and planarity control. Due to the thickness variationsof the wax layer, the wafers would have varying thicknesses and thesurfaces were not always planar.

SUMMARY OF THE INVENTION

The present invention contemplates several improvements in materialpolishing and lapping methods and apparatus. The invention is applicableto the processing of materials where slices or wafers of material areformed and must be lapped or polished on at least one side. Theseprocesses are typically used, as an example, in the manufacture ofsemiconductors and microchannel plates.

The invention will be described as it pertains to the semiconductorindustry for the processing of wafers of semiconductor material andparticularly the lapping of the wafers. However, it is to be understoodthat the invention also applies to the polishing of wafers.

In the preferred embodiment, test slices or wafers have shownsubstantially a 100% yield with no work damage resulting during thelapping operation. Thickness and planarity control are exceptional. Inthe preferred embodiment, the invention does not require the use of asolvent for cleaning the wafers and therefore the cost of the solvent iseliminated as is the adverse effect that result from solvent contactwith the semiconductor material. The use of wax is eliminated in thepreferred embodiment and therefore the process becomes substantiallyfaster as the heating and cooling steps required in the prior art havebeen eliminated. Another totally unexpected result was also realized inthe form of reduced lapping time. Due to the surface characteristics ofthe material used, there is less rotational drag in the lapping machineand higher speeds are achieved resulting in faster lapping. Thus, thepresent invention provides a faster, less expensive method and apparatusfor lapping semiconductor wafers while providing substantialimprovements in the yield.

In the most basic embodiment of the present invention, a liquidphotoresist coating is provided on the face of the semiconductor waferprior to wax mounting the wafer to the lapping plate. The liquidphotoresist coating protects the surface of the wafer from both theabrasive material and solvent during removal and clean-up operation.

In a next embodiment of the invention, a number of wafers are laminatedonto a dry film photoresist disposed on a carrier sheet which issubsequently wax-mounted to the face of the lapping plate. After thelapping process is complete, the lapping slurry is easily washed off thephotoresist and the lapping plate is heated and the carrier is removed.The wafers may be removed from the photoresist by merely peeling thewafers off.

In the preferred embodiment of the present invention, the wafers arelaminated to a dry film photoresist on a carrier sheet which is thenspread over the surface of the lapping plate and held secure to theperiphery of the lapping plate. The carrier sheet may be secured by anelastomeric O-ring or other similar device. A pressed-fit hoop has beenfound to be quite successful since it tends to stretch the photoresistsheet as it is pressed about the plate, thereby removing any wrinklesfrom said sheet and allowing for exceptional thickness control. Thus,the use of a dry film photoresist sheet provides for rapid mounting andremoval of the wafers from the lapping plate without the need forchemical solvent and the heating steps of the prior art. A photoresistsurface protects the face of the semiconductor wafer and work damage issubstantially eliminated, resulting in higher yield of wafers havingcontrolled thickness.

One objective of the present invention is to provide a faster and lessexpensive method for material lapping and polishing.

Another objective of the present invention is to increase the yield ofsemiconductor die after the lapping process.

Another objective of the present invention is to achieve betterthickness control in the lapping operation.

The above mentioned and other features and objects of this inventionwill become more apparent by reference to the following descriptiontaken in conjunction with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a standard lapping plate showing a plurality ofsemiconductor wafers placed on the lapping plate.

FIG. 2 is a sectional view taken along line 2--2 of FIG. 1 showing afirst embodiment of the present invention.

FIGS. 3 and 4 are sectional views showing intermediate steps in themounting of semiconductor wafers to a carrier sheet.

FIGS. 5, 6 and 7 are sectional views showing the various methods andapparatus for mounting the wafers to a lapping plate.

DESCRIPTION OF THE APPARATUS

Referring to FIG. 1, there is shown a plan view of a typical circularlapping plate 10 having mounted on the surface thereof a plurality ofsemiconductor wafers 12. Three adjusting screws 14 are equally spacedabout the periphery of the lapping plate 10, each of said screws havinga diamond point 16 mounted on the upper end. In the standard manner usedin the art, screws 14 are adjusted to control the desired thickness ofthe wafers 12.

Referring to FIG. 2, there is shown how the wafers 12 are mounted to thelapping plate 10 in a first embodiment of the present invention. Thefront face 18 of the wafer 12, the surface in which the components areformed, is coated with a liquid photoresist material 20, such as aphotoresist provided by Hunt Chemical Company known as Waycoat 1C ResistType 3. The photoresist coating 20 protects the face of thesemiconductor wafer during the lapping process and also protects thewafer from the chemical solvent that is used to clean the wafer afterlapping is completed. The photoresist coated wafers are then mounted tothe surface of the lapping plate 10 using the standard wax mountingprocess wherein the lapping plate 10 is heated and a layer 22 of meltedwax is formed over the surface as shown in FIG. 2. After the wax 22 ismelted, the wafers 12 are placed so that the photoresist coating 20 isin contact with the melted wax, which is thereafter allowed to cool andharden to hold the wafers in place on the lapping plate.

The screws 14 are adjusted to provide the desired wafer thickness andthe lapping process is then completed in the standard manner usingstandard lapping materials. After lapping is completed, the excesslapping slurry is washed off and the plate 10 is again heated to meltthe wax 22 so that the wafers may be removed. The photoresist coating 20protects the face 18 of the wafers from damage due to the abrasive andsolvent used in the process. After the wafers are cleaned, thephotoresist coating may be removed using standard photoresist processes.

Referring to FIG. 3, there is shown an improved embodiment of thepresent invention wherein a photoresist in the form of a dry filmphotopolymer on a carrier sheet is used instead of the liquidphotoresist. The semiconductor wafers 12 are placed face-up on a mylartemplate 24 which is formed in a circular configuration simulating thesurface of the lapping plate 10. Preferably, template 24 includesindicia to indicate the proper positioning of the wafers 12 so that thewafers will not interfere with the adjusting screws 14 when placed onthe lapping plate surface. After the wafers are properly positioned onthe template 24 with the front face upward, the wafers are laminated tothe carrier sheet by passing the template and the wafers through a dryphotoresist laminator so that the front surfaces 18 of the wafers adhereto the photoresist layer 26 which is supported by a carrier sheet 28 ofpolyester material such as mylar.

The invention is presently being practiced using a Dynachem CorporationModel 120 Dry Photoresist Laminater operating at a temperature below therecommended temperature (or about 175° F) so that partial adhesion tothe semiconductor wafer is achieved.

There are several dry film polymers that may be used in the practice ofthe present invention, such as photopolymer film resist provided byDynachem Corporation that has a film thickness of 1 to 2 mils formed ona one-mil polyester carrier sheet with a 1-mil polyethylene protectivecover on the top surface. The recommended heating for such a dry filmresist is 235° F ± 10°; however, for purposes of the present invention,only partial heating to about 175° F ± 10° F should be used so thatpartial adhesion is achieved. The partially cured photoresist exhibitshigh shear strength but allows the wafers to be peeled off when tensionis applied. Thus, the material is uniquely adapted for use in lappingmachines where high shear is experienced but not tension.

A dry film photopolymer formed on a carrier was used for the practice ofthe present invention because it was conveniently available and thephotopolymer is totally compatible with the semiconductor material andmay be removed using solutions that are standard in the semiconductorart. The photosensitive characteristic of the material plays no part inthe practice of the invention. The thermoplastic characteristic andcompatibility with semiconductor materials are the important features ofthe polymer used. Other adhesives could be used in the practice of theinvention which would require additional cleaning steps to remove theadhesives in a manner similar to the solvent cleaning required to removethe wax presently used.

After the template 24 is passed through the laminater, the template mayeasily be removed since the mylar does not adhere well to thephotoresist as compared with the semiconductor wafers 12. The wafers 12and carrier sheet 28 are then turned over as shown in FIG. 4 and aresecured to a lapping plate 10 using the standard wax mounting process aspreviously described and now shown in FIG. 5, with the wax layer 30disposed between the carrier sheet 28 and the surface of the lappingplate 10.

After the wafers have been lapped, the lapping plate is washed andreheated to melt the wax layer 30 and the carrier sheet is removed fromthe lapping plate. Due to the surface characteristics of the photoresistthe lapping slurry is easily washed away with little or no abrasiveremaining on or in the surface. The wafers are easily removed from thecarrier sheet simply by peeling them off individually or the entirewafer carrier sheet assembly can be inversely placed on a vacuum tablewhere a vacuum holds the wafers to the table while the carrier sheet ismerely peeled from the wafers. Experience has shown that when properlaminating temperatures are used, no photoresist residue remains on thewafer surface; however, if some residue were to remain, it would beremoved during a subsequent standard clean-up step wherein the wafer iswashed in a hydrogen-peroxide-sulfuric acid solution.

The embodiment of the invention as previously described fully protectsthe surface 18 of the wafers from work damage and also eliminates theneed for solvent cleaning of the wafers since the wax is not in contactwith any portion of the wafer. The abrasive compound does not come incontact with the wax layer 30 and therefore it is possible to re-use thewax layer 30 without the need for cleaning and rewaxing of the surfaceof the lapping plate. Thus, the embodiment previously describedsubstantially increases the yield by reducing work damage and to adegree, reduces the cost of the lapping process since the chemicalsolvents are not required.

It has been discovered that the lapping time can be substantiallyreduced as, for example, from 25 minutes of lapping to 14 minutes oflapping. This is a result of the faster speeds attained by the lappingmachine. Due to the small distance between the lapping plates and theviscosity of the lapping slurry, significant drag was exerted on therotating plates. Due to the surface characteristics of the photoresistand mylar, the lapping slurry does not wet the mylar and less drag isexerted resulting in higher lapping speeds and shorter lapping times.The embodiment previously described still requires the steps of heatingand cooling the lapping plate so that the wax may melt and reharden tosecure the carrier sheet to the lapping plate; therefore, the process isstill rather time-consuming.

It is contemplated that the photoresist could be applied directly to thelapping plate and fully cured with the carrier sheet removed. The waferscould then be applied to the photoresist with moderate heat so thatsufficient shear strength is provided while allowing for subsequentremoval by lifting of the wafers. Developing solution would then be usedfor subsequent cleaning of the plate surface. A moderate amount of heatmay be applied to soften the photoresist prior to removal of the wafers.

Referring to FIG. 6, there is an improved embodiment of the presentinvention wherein the need for wax is totally eliminated. The lappingplate 10 has been modified to provide a beveled edge 32 and a groove 34about the entire periphery of the lapping plate. The threaded openingsthat receive the adjusting screws 14 were also modified to provide acircular recessed surface 36 about the threaded opening and a shoulder38 formed approximately half-way through the threaded opening with thethreads terminating at the shoulder 38, and the opening having a largerdiameter above said shoulder. A pair of small diameter openings 40 areprovided about each of said adjusting screws and extend entirely throughthe lapping plate 10, said openings terminating within the recessedcircular surface 36. The purpose of the openings 40 will be discussedsubsequently.

In practicing this embodiment of the invention, the wafers 12 aremounted to a carrier sheet as described and shown in FIGS. 3 and 4.Three holes are then formed in the carrier sheet and are located tocoincide with the three adjusting screws 14. The holes may be formed inany of a number of ways; however, it has been found that the use of ahot wire is particularly useful since the melting carrier sheet forms abead about the opening as it forms. The carrier sheet is then spreadover the surface of lapping plate 10 so that the holes in the carriersheet register with and fit around the adjusting screws. Plastic plugs42 are then pressed into the space about the adjusting screws, saidplugs have flanged upper surfaces 43 that engage the carrier sheet andhold it in place against the recessed surfaces 36. After the plugs 42are properly inserted, the carrier sheet is folded down over the bevelededge 32 and groove 34 and a tight-fitting elastomeric O-ring 44 is usedto hold the periphery of the carrier sheet in place about the groove 34of the lapping plate 10.

It was discovered that the beveled edge 32 was useful in this embodimentsince the carrier sheet tended to wrinkle and fold, forming pleats as itwas turned down over the edge of the lapping plate. The pleats wouldhave a substantial thickness extending above the lapping surface andinterfering with the lapping operation and in most instances, being cutor severely abraded during lapping. Through the unique use of thebeveled edge, the pleats form over the beveled surface and therefore donot interfere with the lapping operation. After the lapping iscompleted, the elastomeric ring 44 is removed and the plugs 42 areejected using a bifurcated tool having a pair of pins that extendthrough the openings 40 to push plug 42 out of the recess. The wafers 12are thereafter removed from the carrier sheet by merely peeling, and nosolvent is required for the process. This embodiment of the inventionprovides all of the advantages of the previously mentioned embodimentsand in addition requires no wax or solvent. Since wax is not required,excellent thickness control and planarity are realized. The dry filmpolymer has a fairly consistent thickness, thereby allowing forexcellent thickness control of better than 0.5 mil tolerance.

Referring to FIG. 7, there is shown the preferred embodiment wherein thelapping plate 10 is formed with an annular shoulder 46 formed about theperiphery thereof, said shoulder having a plurality of notches 48 formedtherein, the purpose of which will be described subsequently. An annularring 50 is press-fit about the lapping plate 10 to tightly engage thecarrier sheet between the ring 50 and the lapping plate. It has beendiscovered that a ring formed of a plastic material, such as Delrin, isentirely satisfactory for the practice of the present invention. Ring 50may be manually pressed in place and subsequently removed by prying witha device, such as a screwdriver. One advantage of this embodiment overthat shown in FIG. 6 is that it may be practiced with machine operateddevices. It is contemplated that the ring be pressed on using a machineoperated press and subsequently removed using a machine operated pullerhaving fingers that fit into notches 48 and engage a lower surface ofring 50 for extracting the ring from about the lapping plate. The plugs42 are inserted and removed as previously described for the embodimentof FIG. 6. It has been discovered that by using a tight-fitting ring 50that is press-fit over the lapping plate 10, the carrier sheet isstretched slightly so that no wrinkles form on the surface of thelapping plate.

Using the present invention, substantially all of the work damageassociated with the prior art has been eliminated and substantially 100%yields have been achieved during the lapping process. The mounting andremoval procedure has been significantly simplified in that thetime-consuming heating and cooling steps have been eliminated and thereis no longer a need for the use of solvent to remove the wax from thelapped wafers. Thus, the cost of the operation has been reduced in thatthe solvent has been eliminated and the cost of energy for heating thelapping plate has also been eliminated. The time required to perform theoperation has been substantially reduced since the wafers are almostinstantaneously mounted to the lapping plate and long heating andcooling periods are not required. A significant advantage of the presentinvention is the substantial increase in yield of semiconductor diewhich, in itself, substantially reduces the overall cost of productionof semiconductor devices. The invention also allows for better controlof wafer thickness and planarity so that better quality die areproduced.

While the present invention has been practiced using a photoresist andpreferably a dry film photopolymer on a carrier, it is to be understoodtht the invention is not limited to the use of these specific materials.Other types of dry film polymers or thermoplastic adhesives could alsobe used in the practice of the present invention.

The advantage of a photosensitive polymer is that it breaks down whenexposed to light and may then be washed away using developer rather thanstrong solvent solutions that would be required for other types ofadhesives. It is clearly advantageous to avoid the need for a separatecleaning step to remove adhesive residue from the wafer. The materialused must also have a high shear strength but low tensile strength sothat the wafers may be peeled off without damage. Thermosensitiveadhesives are preferred as opposed to adhesives that require theapplication of a solvent of soften the adhesive prior to applying thewafers. The photoresist type of dry polymer film is used because it is aconvenient material readily available to those in the electronics artand is compatible with standard semiconductor processes.

While the principles of this invention have been described above inconnection with specific embodiments, it is to be understood that thisdescription is made only by way of example and not as a limitation onthe scope of the invention as set forth in the objects and in theaccompanying claims.

What is claimed is:
 1. A method for mounting material to be processed toa supporting surface of a processing apparatus wherein a selectedsurface of the material is to be protected, comprising the stepsof:applying a protective coating to the selected surface of thematerial; and disposing a temperature-sensitive material between theprotective coating and the supporting surface for securing the coatingto said surface, whereby the protective coating protects the selectedsurface of the material during the processing and from solvents used toremove the temperature-sensitive material.
 2. A method as described inclaim 1, wherein the temperature sensitive material comprises a wax andthe disposing step includes the steps of:applying the wax to thesupporting surface; heating the wax to its melting point; placing theselected surface with protective coating on the melted wax; and coolingthe wax until it solidifies, thereby securing the material to thesupporting surface.
 3. A method for mounting a semiconductor wafer tothe surface of a lapping plate in such a manner as to protect the frontface of the wafer during the lapping operation, comprising the stepsof:laminating the front face of said semiconductor wafer to a dry filmphotopolymer disposed on a carrier sheet; and securing said carriersheet over the surface of said lapping plate.
 4. A method for mountingmaterial to be processed to a supporting surface of a processingapparatus wherein a selected surface of the material is to be protected,comprising the steps of:applying a layer of polymer material to theselected surface of the material; and securing said layer of polymermaterial to the supporting surface.
 5. A method for mounting material tobe processed to a supporting surface of a processing apparatus wherein aselected surface of the material is to be protected, comprising thesteps of:applying a layer of photoresist material to the selectedsurface of the material; and securing said layer of photoresist materialto the supporting surface.
 6. A method as described in claim 5, whereinthe layer of photoresist material is secured to the supporting surfaceusing wax.
 7. A method for mounting material to be processed to asupporting surface of a processing apparatus wherein a selected surfaceof the material is to be protected, comprising the steps of:applying adry film polymer disposed on a carrier sheet to the selected surface ofthe material by heat laminating the polymer to the selected surface; andsecuring the carrier sheet to the supporting surface.
 8. A method asdescribed in claim 7, wherein the material to be processed comprises awafer that is attached to the dry film polymer and the carrier sheet bypassing the wafer, dry film polymer and the carrier sheet through a heatlaminating apparatus.
 9. A method as described in claim 8, wherein thewafer is first placed on a template with the selected surface facingupwardly and the template and wafer are passed through the laminatingapparatus for applying the dry film polymer to the selected surface ofthe wafer.
 10. A method as described in claim 7, wherein the carriersheet is mounted to the supporting surface utilizing a wax.
 11. A methodas described in claim 7, wherein the carrier sheet is mounted to thesupporting surface by folding the sheet downwardly about a periphery ofthe surface and applying a securing means about the periphery of thesurface to securely engage the carrier sheet about the periphery of thesupporting surface.
 12. A method as described in claim 11, wherein thecarrier sheet is secured about the periphery of the supporting surfaceby applying an elastomeric ring about said periphery.
 13. A method asdescribed in claim 11, wherein the supporting surface is circular and isdefined by a cylindrical peripheral surface and the carrier sheet isfolded downwardly about the cylindrical surface and is secured theretoby a press-fit annular ring.
 14. A method as described in claim 7,additionally comprising the steps of:forming a plurality of openings insaid carrier sheet; orientating said openings so as to be inregistration with adjusting screws protruding above the supportingsurface; and securing the periphery of said openings to the supportingsurface.
 15. A method as described in claim 7, wherein the polymer is aphotopolymer.